1. Field of the invention
The present invention relates to a solid state imaging device having, for example, a concave lens structure formed therein and its manufacturing method.
2. Description if the Related Art
Recently, in a color solid state imaging device, as the device is down-sized, a so-called on-chip structure, in which a color filter is formed within the device and further a microlens is formed on the color filter, is employed, whereby an incident light is converted by the microlens to improve the sensitivity at a sensor or light receiving portion.
Further, there is proposed such a solid state imaging device in which a second lens structure having a light converging characteristic is provided in the solid state imaging device having the above-mentioned on-chip structure between the microlens on its surface and the light receiving portion.
As the second lens structure, there is proposed a concave lens structure in which the boundary face between two layers having different refractive indexes, for example, is a concave face on which a concave lens is formed, and so on.
FIG. 1 is a schematic diagram showing an example of a solid state imaging device 50 in which a concave lens structure is formed between a surface layer and a light receiving portion.
In the solid state imaging device 50, a sensor (light receiving portion) 52 is formed in the semiconductor substrate 51 and a transfer electrode 54 is formed on the semiconductor substrate 51 except for the light receiving portion 52 through a gate insulating film 53. A light shielding film 56 is formed on the transfer electrode 54 through an intra-layer insulating film 55. The light shielding film 56 prevents a light from being incident on the transfer electrode 54. An opening is formed through the light shielding film 56 at its portion on the light receiving portion 52 so that the light is incident on the light receiving portion 52 through the opening.
For example, a BPSG film 57 is formed to cover the light shielding film 56. This BPSG film 57 has on its surface a concave and a convex corresponding to a step formed by the light shielding film 56 or the surface portion of the BPSG film 57 just on the light receiving portion 52 becomes a concave portion.
On the BPSG film 57, there is formed a high refractive index layer 58 which is made of, for example, a SiN film (refractive index n=1.9.about.2.0) or the like to form a concave lens structure (so-called intra-layer lens) therein. The upper surface of the high refractive index layer 58 is flattened and a color filter 60 is formed on the flattened surface through a passivation film 59. On the color filter 60, a microlens is formed 61.
In this case, in order that a light incident on the concave lens surface, namely on the boundary surface between the two layer or BPSG film 57 and the high refractive layer 58 is converged on the light receiving portion 52, it is necessary to adjust the relation between the refractive indexes of the BPSG film 57 and the high refractive index layer 58.
In general, in consideration of the concave lens, in order to converge the incident light on the light on the light receiving portion 52, such an adjustment is carried out that the refractive index of the high refractive index layer 58, which is an upper layer than the BPSG film 57 with the lens surface as the boundary, is made higher than the refractive index of the BPSG film 57.
When a light is incident on the concave lens surface in an inclined direction, however, dependent on the incident angle of light, there is such a case that the light will be incident on the concave lens surface at a large angle which will not occur in a structure having formed no concave lens structure.
Therefore, it will be predicted that a total reflection of light occurs on the concave lens surface dependent on the incident angle and hence there may be such a fear that the improvement of sensitivity becomes insufficient.